1. Field of the Invention
This invention relates in general to electronic circuitry to protect excessive currents flowing through a power transistor, and in particular to a circuit that monitors the voltage across the transistor to determine excessive current.
2. Description of the Prior Art
Large capacity power transistors are used in variable speed drive systems for large alternating current (AC) motors. In a variable speed drive system of this type, three phase AC power is rectified into positive and negative DC rails. The three power lines for the three phase AC motor are connected to these rails through power transistors. When a power transistor is on, it will supply voltage from one of the rails to the motor. Control circuitry is used to control the frequency and sequence of the operation of the power transistors.
Power transistors are normally operated in a saturated condition. That is, increasing the current to the base beyond the saturated point will not result in any more increase in current through the collector. If the collector current becomes excessive relative to the base current, this indicates that the transistor is coming out of saturation. Normally this happens only if a malfunction is occurring. Perhaps more than one of the transistors are erroneously conducting at the same time.